This invention relates generally to optical sensing of various parameters, and more particularly to optical sensing that includes the use of luminescent material. This invention has two principal aspects, one of which pertains generally to temperature measurement, and the other of which pertains to the general measurement of a second parameter, such as pressure, force, acceleration, refractive index, or vapor pressure, along with the measurement of temperature.
With regard to the first principal aspect of the present invention, as background, there are a large number of instances where accurate determination of the temperature of a solid is desired or necessary. For example, solid material being processed often requires that its surface temperature be known in order to adjust the parameters of the processing steps. A specific example is in the fabrication of electronic semiconductor devices wherein it is desirable to know the temperature of a surface of semiconductor wafers or other solid materials as they are being processed. Since many semiconductor processing steps are now conducted in a vacuum chamber, rather than in an oven or furnace, the difficulties of measuring surface temperatures are increased. In many cases where it would increase efficiency, quality of the resulting product, or reduce costs, surface temperature should be measured but cannot be by existing techniques because of the difficulty, expense or inaccuracy.
One technique that is used for surface temperature measurements is to attach a small thermistor or a thermocouple to the surface, or to deposit a resistive film on it. This is a very tedious operation and cannot be utilized in routine production applications, or in electrically or chemically hostile environments.
Infrared (I.R.) radiometry is an alternate, non-contact technique for measuring surface temperature by observing the infrared energy emitted from the surface of interest. This technique, however, requires that the emissivity of the surface being measured be known with great accuracy. Otherwise, the temperature measurements are not reliable. Unfortunately, it is difficult to accurately know the emissivity of the surface, particularly in applications where it is changing as a result of processing that surface by etching, coating and the like. In an application to semiconductor wafer processing, it is hard to use I.R. because of the transparency of most semiconductor wafers to those wavelengths, limited accessibility to the chamber in which the processing is taking place, and its poor sensitivity and accuracy at typical wafer processing temperatures.
A more recent technique utilizes luminescent materials that, when properly excited to luminescence, emit radiation with a characteristic that is proportional to the phosphor's temperature. There are two primary categories of luminescent temperature sensing techniques that are currently receiving attention. One such technique involves the detection of the intensity of emitted radiation from a luminescent material in two different wavelength ranges and then ratioing those intensities to obtain a value proportional to temperature. An example of this technique is given in U.S. Pat. No. 4,448,547-Wickersheim (1984). In a second technique, the luminescent material is illuminated with a pulse of excitation radiation and then the decay time, or a quantity related thereto, of the luminescent after-glow radiation is measured. Examples of this technique are given in U.S. Pat. No. Re. 31,832-Samulski (1985) and U.S. Pat. No. 4,223,226-Quick et al. (1980), and in allowed copending application Ser. No. 787,784, filed Oct. 15, 1985, now U.S. Pat. No. 4,652,743 and assigned to the assignee of the present application. The temperature of the luminescent material sensor is determined by either technique, thus providing a determination of the temperature of the environment surrounding the sensor.
These luminescent techniques have been used in two general ways for measuring surface temperature. A first is to attach a layer of the phosphor material in direct contact with the surface whose temperature is to be determined, one form being to paint onto the surface a transparent binder carrying phosphor particles. The phosphor emission is viewed by an optical system positioned some distance from the surface. A shortcoming of this technique is that it is often difficult to implement for many applications since the attachment of the phosphor to the surface may be too permanent and/or there may not be a necessary clear optical path between the phosphor and the optical elements. This technique does have the advantage of detecting temperature in a remote manner and minimizing any surface perturbation, and, for that reason, is advantageous for other applications.
A second category of luminescent sensor surface measurement techniques utilizes a phosphor sensor on the end of an optical fiber. This technique has the advantage of only a temporary contact with the surface being required, but has a disadvantage that, in applications where extremely accurate temperature measurements are required, the optical fiber carries away heat from the surface and also presents an undesired thermal mass that must be heated by the surface being measured. These factors cause the resulting temperature measurements to be offset from the true temperature of the surface and may also slow down the time response of the sensor.
Therefore, it is a primary object of the present invention to provide a surface temperature measurement technique and device that minimizes these difficulties.
With regard to the second principal aspect of the present invention, as background, the measurement of various physical parameters other than temperature, such as force, pressure, acceleration, refractive index and vapor pressure with optical devices is very desirable for many applications where electrically conductive elements must be avoided. For example, one such application of pressure measurement includes an environment of highly volatile liquids or gases where electrical leakage or discharge may be a serious hazard. Medical applications are numerous, especially where miniature catheters are required to measure body fluid pressure in a specific organ or blood vessel. Voltage breakdown during defibrilation may be destructive to many conductive types of pressure transducers and may also create undesirable electrical currents in the patient. There is also the consideration of excessive pressure overload damaging the transducer.
Some of the many techniques used for pressure monitoring range from ceramic piezo-electric discs which generate a voltage when stressed, to similar silicon devices with resistors deposited in many different fashions to form an electrical resistive network which, when deformed, predictably changes the resistance ratio. Shear effect in semiconductors is also being used at this time Older methods, such as diaphragms with strain gauges or beams attached, are also still widely used. All of these techniques require special insulation and protection methods, both mechanical and electrical, which make the product difficult to manufacture and then still presents some degree of the risks mentioned above.
As a result, optical techniques are also being used to measure various physical parameters other than temperature in order to overcome the operational and structural problems described above. The usual optical technique uses a sensor from which an optical signal of the parameter being measured is communicated along an optical fiber. One example of such an optical fiber sensor is a reflective diaphragm whose position is proportional to the condition being measured, such as force or pressure, and that position modulates the intensity of the light signal passed through the sensor by the optical fiber communication medium. The optical signal proportional to the parameter being measured is then detected at an opposite end of the optical fiber medium. Other fiber optic sensors of force or pressure include those which use beams, the compression of fibers between two plates, vibrating crystals which modulate reflected light, and coherent-light phase shift and amplitude modulation effects. Similar types of sensors have been employed to measure other physical parameters than force or pressure, such as displacement or alignment of an element, mass or weight, magnostrictive or electrostrictive effects, and the presence of contamination.
The manufacture of all these types of sensors generally require delicate mechanical structures that are time consuming to assemble and test. Therefore, it is another object of the present invention to provide a simple, sturdy and economical technique for measuring such parameters.
It is a further object of the present invention to provide an optical measurement technique and device capable of simultaneous measurement of temperature and a second physical parameter.